This invention relates to a process for producing a printed circuit board by the additive process employing a photoresist. More particularly, it relates to a process for producing a printed circuit board, which is suitable for the formation of microwiring by the additive process.
Conventionally, as a process for producing a printed circuit board at a low cost, there has been known a so-called additive process wherein a negative pattern is formed by using a plating resist and a circuit is formed by chemical copper plating on the areas not covered by the plating resist. For the dense packaging of such an additive-type printed circuit board, it is effective to utilize a photoresist by employing a photosensitive resin as a plating resist and forming an image by light. In the case of the additive process, as distinguished from the etched foil process or pattern electroplating process in view of process steps, the resist can be left as such after completion of the plating on circuit areas and, hence, the step of resist stripping can be omitted. It is also possible by leaving the resist as such in the substrate areas not carrying conductor wiring to obtain flat surface with no difference between the surface level of conductor areas and that of other areas. There is another advantage in that the perfect coverage of the conductor surface becomes easier in forming a permanent mask for the protection of conductor surface. However, to utilize a photoresist as permanent resist, it is necessary for the photoresist to have, in addition to those image forming characteristics which are generally required for a photoresist, other thermal, chemical, and mechanical stabilities such as soldering resistance, solvent resistance, and heat shock resistance. A process for making a printed wiring board using a photosensitive resin which meets the above requirements is described in Japanese Pat. application "Kokai" (Laid-open) No. 100,490/83. In forming a resist pattern by use of a photosensitive resin, an exposure to form the image is insufficient for complete curing of the resin and it is difficult to meet the above requirements of thermal, chemical and mechanical stabilities. As a consequence, it has generally been believed to be effective that after having been formed by development, the resist pattern is further subjected to treatments such as heating, exposure and the like to cure completely the photosensitive resin.
When a photoresist is used in the additive process, it has been general practice to carry out, prior to the chemical copper plating, the post exposure and post heat treatment in succession to the imagewise exposure and development as a sequence of image forming process steps. The reason for this is such that it was thought necessary to enhance the plating resistance of the photoresist by post exposure prior to the chemical copper plating. However, it has become apparent that if such a technique is used in the case of full additive process by carrying out the post exposure prior to the chemical copper plating, there arise such problems that (1) plating void is likely to result and (2) the adhesion strength between the deposited copper film and the substrate becomes decreased.